1. Field of the Invention
The present invention relates to a brushless motor driving system for driving a brushless DC motor by sensing a rotor position of the rotor thereof, and in particular, a system and a method for sensing a rotor position of a brushless DC motor based on voltage induced in a phase coil of the brushless motor.
2. Description of the Related Art
JP-A-62-123979, JP-A-9-266690 and JP-A-7-288992 respectively disclose brushless DC motor driving system, in which brushless DC motors are operated based on the rotor position of their rotor that is detected by phase voltage induced in stator coils thereof.
The brushless DC motor disclosed in JP-A62-123979 may be used for a blower motor having a cooling fan, as a load, to cool a radiator of an automobile in a similar manner as shown in FIG. 11 of this application. That is, the brushless DC motor driving system includes a brushless DC motor 3, an inverter 4, a control unit 7, a gate driver 8 and a rotor position detecting circuit 9. The inverter 4 is constructed of a three-phase bridge circuit of six power metal oxide semiconductor field-effect transistors (MOSFETs) 5a, 5b, 5c, 5d, 5e and 5f. The inverter 4 has three output terminals respectively connected with three phase coils 6U, 6V, 6W of the DC motor 3. The inverter 4 is controlled by the control unit 7 via the gate driver 8, which provides the gates of the MOSFETs 5a, 5b, 5c, 5d, 5e and 5f with drive signals, so that the inverter outputs PWM signals to the brushless DC motor 3. The rotor position of the rotor of the brushless DC motor 3 is detected by the rotor position detecting circuit 9 and sent to the control unit 7. The rotor position detecting circuit 9 includes three low-pass filters 10U, 10V, 10W, each of which is constructed of a capacitor Cu, Cv or Cw and a resistor Ru, Rv or Rw, three buffer amplifiers 11U, 11V, 11W and three comparators 12U, 12V, 12W. The input terminal of each of the low-pass filters 10U, 10V, 10W is connected to a junction of voltage dividing resistors R1U-R2U, R1V-R2V, or R1W-R2W which are respectively connected between the output terminals of the inverter 4 and a ground.
When the motor 3 is operated, the control unit 7 gives the inverter electric power of a preset pattern to provide PWM switching signals. When the motor 3 is operated, phase voltage is induced in each of the stator coils 6U, 6V, 6W to be superimposed over PWM switching signals, as shown in FIG. 12A, in which only the voltage waves in U-phase stator coil 6U is shown. The low-pass filters 10U, 10V, 10W remove the switching signals and shapes up the phase voltage to output sinusoidal voltage signals as shown in FIG. 12B. Then, the comparator 12U, 12V and 12W respectively compare the sinusoidal voltage signals with a virtual neutral voltage level and output rectangular rotor position signals, as shown in FIG. 12C.
The control unit 7 is given control signals from an outside electric control unit to control the duty ratio of the PWM switching signals. The control unit 7 also sets switch timing to form the driving signal to be outputted to the gate driver 8.
When the induced voltage passes one of the low-pass filters 10U, 10V, 10W, phase delay increases as the rotation speed of the motor (or the rotor) increases, as shown in FIG. 15B. Although the phase delay becomes stable at the phase delay of 90 degrees if the time constant of the low-pass filters increases as shown in FIG. 15B, the gain of the low-pass filter decreases as the time constant thereof increases, as shown in FIG. 15A. Further, the phase delay changes when the temperature of the low-pass filter, which is usually disposed near the radiator of an automobile, changes. Therefore, it is difficult to control the phase delay to a fixed phase angle, such as 90 degrees.
A brushless DC motor driving system disclosed in JP-A-9-266690 also has a similar rotor position detecting system in construction except that the low-pass filters 10U, 10V, 10W are omitted, as shown in FIG. 13. This control unit detects zero-cross points of the induced voltage waves by comparing the induced voltage with a reference voltage level and provides turn-on timings that are different in phase angle of 30 degrees from each other, as shown in FIG. 14.
However, it is impossible to detect the zero-cross points when the rotation speed of the brushless DC motor becomes higher than a certain level due to current flowing through diodes each of which is connected across one of the MOSFETs of the inverter, as shown in FIG. 16A. It is also impossible to detect the zero-cross points when the phase of the phase signal is advanced by more than 30 degrees in electric angle, as shown in FIG. 16B.
The control unit of a brushless DC motor driving system shown in JP-A-7-288992 detects third-order harmonic wave components by comparing the mechanical neutral point of the motor and a virtual neutral point formed by a resistor circuit, thereby detecting the rotor position of the rotor.
However, it is difficult to apply this system to a system that includes a brushless DC motor having delta-connected phase coils because it is necessary to connect a rotor position detecting circuit with the mechanical neutral point, which does not exist in the motor having the delta-connected phase coils.